A known form of flow-monitoring apparatus involves electrical conduction in flowing liquid, disclosed in British Pat. No. 804,333 dated Nov. 12, 1958. There, two electrodes are exposed to flowing water, a reference electrode and a sensing electrode. The electrodes and the water between them form an arm of an alternating-current Wheatstone bridge. The a-c exited electrodes are of gold or gold-plated to resist corrosion. The sensing electrode is a fine wire having its exposed end disposed close to the path of a flow-driven propeller's blade tips. Another arm of the bridge may be a resistor, or it may be the water's impedance between the reference electrode and a third electrode downstream of the propeller. The output signal, when demodulated, is a series of sharp pulses. Performance depends on the propeller's blade tips being critically close to the fine sensing electrode for largely blocking conduction briefly as each blade tip passes the fine electrode.
Another form of flow sensor that also depends on electrical conduction in the flowing liquid is disclosed in my U.S. Pat. No. 4,333,354. There, multiple electrodes are exposed to water or other liquid having limited electrical conductivity. Alternating-current excitation is applied to the electrodes (direct-current also being mentioned) so that a pattern of current paths forms in the liquid. Vanes of a flow-activated rotor move successively into position clear of the current paths between two electrodes and into position wherein a substantial portion of the current paths would extend through the thickness of the vanes but is suppressed thereby. That performance depends on the described relationship between the electrodes and vanes.
My U.S. Pat. Nos. 4,399,696 issued Aug. 23, 1983 and 4,535,637 issued Aug. 20, 1985 also involve vanes of a flow-activated rotor alternately becoming a barrier to current paths in the liquid and freeing the current paths in providing a sensed signal. In those patents, an a-c or pulsed excitation pattern of current paths is variably affected by rotor vanes; the sensed signals are phase-compared to the excitation signal for producing flow-representing pulses.
Thus, alternating-current excitation has been used in flow monitors having electrodes exposed to liquid that exhibits some conductivity. In such flow monitors, the excitation frequency is much higher than the frequency of the vanes passing the sensing electrode(s) at the maximum flow rate, so that such apparatus routinely includes its own excitation-frequency generator. Flow monitors are often installed in locations where electrical service is not available, so that batteries of the long shelf-life type are used for providing energy to operate the circuit. The electrical energy requirement of the excitation generator and the demodulation circuit usually represents a large proportion of the total energy needed by the flow-monitoring apparatus. Where a battery is used, a circuit that depends on a local carrier-frequency signal generator leads to the expense of an unduly large and costly long-life battery, e.g. lithium cells, or more-or-less frequent service calls are needed for replacing the battery. Avoiding a battery entails the cost of providing electrical service to the flow-monitoring apparatus.
A still further consideration of flow-monitoring apparatus of the kind that involves electrical conduction through the flowing liquid is that the apparatus, as constructed, may be used with liquid whose resistivity may be any sustained value in a wide range. For example, sea water has vastly greater conductivity than tap water, and even the conductivity of tap water differs at different locations and from time to time. One effort toward adapting flow-monitoring apparatus to liquids of various resistivities is found in British Pat. No. 804,333 where a Wheatstone bridge includes what may be called a measurement arm comprising the sensing electrode and a reference arm with a separate electrode. A third electrode common to both arms is also used. The resistances of both arms vary together, in proportion to the resistivity of the flowing liquid. However, in the Wheatstone bridge, reduced resistance of the vane-responsive arm is accompanied by reduced signal amplitude representing the flow. In addition, the requirement of three electrodes exposed to the liquid is a factor that complicates and involves significant expense to the structural portion of the flow-monitoring apparatus.
In my U.S. Pat. No. 4,535,637, supra, the disclosed flow-monitoring apparatus has a long-time-constant feedback circuit between the demodulated output circuit and the sensed signal input circuit. That apparatus implicitly involves a-c or pulsed excitation and it also depends on the provision of both excitation and sensing electrodes.